Millions of Americans rely on dietary supplements to stay healthy. About one-half of adults regularly take them, spending more than $20 billion annually to manage and prevent chronic diseases, such as heart disease, diabetes, osteoarthritis, various cancers and osteoporosis.

Do they work to ward off diseases?

No, and they may even increase the risk of death, according to a new study published last week in the journal Archives of Internal Medicine.

Researchers compared the use of vitamin and mineral supplements to death rates in more than 38,000 older women participating in the Iowa Women’s Health Study (IWHS). Surveys were conducted over nearly 20 years, tracking self-reported supplement use in 1986, 1997 and 2004.

Two-thirds of women reported taking at least one dietary supplement each day at the start of the study. The proportion of users increased to 85% after nearly 20 years. Over one-quarter of women reported using four or more supplements, and vitamin use contributed significantly to the intake of several key nutrients. Death rates were then observed between women who took supplements and those who didn't.

The conclusions were startling. In older women, several commonly used vitamin and mineral supplements were associated with increased death rates, with the highest mortality rate linked to supplemental iron, the researchers found. The only supplement linked with decreased risk of death was calcium, which presumably enhanced bone health.

So should you empty your medicine cabinets of all vitamins, save calcium?

Not just yet. While not a ringing endorsement for vitamin supplementation, consider the study’s limitations before you stop supplementing.

This wasn’t a prospective, randomized trial but a cohort study (supplement users compared to non-supplement users) that relied on a self-administered questionnaire. The study was originally designed to examine the associations between several host (health status), dietary, lifestyle factors and the incidence of cancer in post-menopausal women. The dietary supplement portion of the Food Frequency Questionnaire used in the study wasn’t validated separately, potentially limiting the validity of the observed results.

Neither blood lipid levels nor blood pressure were measured in the survey, and blood cell counts weren’t performed. So, for example, the use of iron supplements may have been used to treat illnesses that result in anemia. These include chronic disease, major injury and operations that correlate with higher mortality rates. The authors could not exclude the possibility that some supplements were taken in response to symptoms or clinical disease not picked up in the questionnaires.

The study didn’t collect data regarding nutritional status or detailed information of supplements used. For example, different forms of vitamin D, cholecalciferol (D3) or ergocalciferol (D2) weren’t distinguished. Several recent publications support the use of supplemental D3 for women with low vitamin D levels. Also, the study population consisted of 99.2% white women. Therefore, applying the results to other populations, ethnic groups, or men is questionable.

Despite all of these reasonable objections to the study limitations, it’s important to acknowledge the impressive size and duration of the study. Elderly women and their caregivers should consider its findings when deciding on the risks, cost and benefits of dietary supplements.

While the study didn’t produce a definitive conclusion, it’s certainly not a clarion call for increased government regulation advocated by some.